Scientists already knew that vampire bats have snakelike pit organs in their faces that point the mammals to the juiciest parts of their prey—the veins. But it was unknown how the predators located those choice biting spots.

Now a study has shown that bats have evolved special facial nerves that can detect body temperatures as low as 89.6 degrees Fahrenheit (32 degrees Celsius).

The vampire bat is "clearly adapted in a lot of unusual ways for a very unusual lifestyle—this is one more example," said study co-author Nicholas Ingolia, a genomics researcher at the Carnegie Institution for Science in Baltimore, Maryland.

People have a similar heat-sensitive channel, but it's only activated by painfully hot stimuli, such as touching a hot stove. The vampire bat, the study found, has two forms of heat-sensitive channels, one to sense painful heat—like us—and another to zero in on its prey's veins.

Such an evolutionary adaptation—known previously only in three snake species—is especially crucial for the vampire bat, which needs a blood meal every one or two days to stay alive.

The vein-sensing ability is "an extreme version of an existing trait, Ingolia said. Other bats have the gene for this hypersensitive channel, but only the bloodsuckers' bodies appear to activate the gene.

In the study, Ingolia and colleagues isolated the nerve cells that travel to the pit organs in the bat's face and compared them with the sensory nerve cells that go to the rest of the bat's body.

The nerve cells were anatomically different from the regular, pain-sensing nerve cells, which means the vein-sensing cells convey separate information, Ingolia said.

The team also looked at vampire bat genes and found the same heat-sensing gene is found throughout the body, but the gene results in a different type of nerve cell in the pit organs.

The genetic work also revealed more about vampire bats' relationships to other mammals. Despite their ratlike appearance, the bats proved surprisingly similar, genetically speaking, to cows and horses, according to the study, to be published tomorrow in the journal Nature.

The horse connection might not be so surprising to anyone who's seen a vampire bat in action.

Hunting under the cover of darkness in the American tropics, the bat gallops, horselike, along the ground, approaching its prey on all fours. Razor-sharp teeth slice into the victim's vein, and the bat laps up the trickling blood with its tongue.